1. Field of the Invention
The invention relates in general to audio-visual devices; and more particularly to audio-visual devices for providing audio output and synchronized control of an optical projector in response to prerecorded signals on a magnetic tape.
2. Description of the Prior Art
Audio-visual devices for providing audio output and control of an optical projector having a single frame advance mechanism in response to prerecorded signals on a magnetic tape are generally known. A magnetic tape for such a device contains a sequence of audio portions each of which portion corresponds to a frame to be shown by the optical projector. Each such audio portion is preceded in time sequence by a visual advance control signal on the tape. Once the projector and audio signals have been initially sychronized, during a normal automatic forward operation such a device provides audio output in response to the audio portions detected from the tape and control to advance the single frame advance mechanism of the projector one cycle in response to each visual advance control signal detected from the tape such that the audio output and display of the optical projector remain in synchronization as the tape is played by the device.
An important feature in such a device is the capability for the operator to override the normal automatic forward operation by actuation of manual switching to command the device to drive the tape at increased speed in the reverse direction with the audio muted to provide a review mode. U.S. Pat. No. 3,756,715 describes a device having an auto-manual switch which provides normal automatic forward operation in the auto position. When the switch is changed to the manual position, the visual advance control signals on the tape are used by the device to stop the tape only. Once the tape is stopped in manual mode, the operator must initiate a command to cycle the projector one cycle in an appropriate direction by pressing a separate forward or a separate reverse switch with the next visual advance control signal on the tape in the appropriate direction again stopping the projector. Such a device is not as convenient as may be desired in manual override mode, however, because it requires a separate command from the operator for each projector cycle and requires the operator to wait until the tape stops before again activating the device.
Another known techinque for providing a review mode feature in such a device is to override the normal automatic forward operation by actuation of a manual switch to command the device to drive the tape at an increased speed in reverse direction with the audio output muted and the optical projector being cycled in response to visual advance signals detected from the tape. Telex Corporation has marketed a device having a manual override key with a first position that enables normal automatic forward operation and a second position that enables the review mode. Such device in response to movement of the manual override switch from the second to the first position resumes normal automatic forward operation. Very often the audio resumes in the midst of an audio portion which is confusing to a person observing the presentation. Also, such device often gets out of synchronization such that the audio output portion for one frame is presented with the visual display of a different frame to produce an embarassing and distracting effect.